The induction balanced metal detector search coil is typically an air core transformer with an outer large primary coil for inducing a magnetic field in the ground to be investigated and a smaller secondary coil producing an output signal that is altered by eddy current objects, such as coins, within the field area.
However, the secondary signal may be several volts from the induced current from the primary, which masks the usual tiny millivolt signal change due to the coin being sought. Usually, various methods are employed to reduce the induced secondary signal to nil by achieving an “induction balance”. One method is to have a third coil or second primary that is tightly coupled to the secondary coil, but of opposite phase to the main primary such that the net voltage in the secondary is nil, until the field is disturbed by the coin or other eddy current object. Typically, this three-coil arrangement is arranged as three concentric coils. A near perfect null is achieved by physically adjusting the last turn of the second primary and then anchoring it in place.
Another method of achieving the induction balance and getting a good null is by having one primary and a slightly overlapping secondary that is critically located where the null is a minimum.
Both methods suffer from increases in the null voltage due to minor mechanical shifts in the coil positions, both in manufacturing and due to aging.
Previous methods of achieving an electronic null have utilized feedback from the large primary driving signal through a variable gain device, such as an OTA (operational transconductance amplifier) to the secondary coil signal. But this method introduces noise because the canceling voltage is not from the inductive coupling of the air core transformer.
Needs exist for improved induction balance and improved nulling of the primary coil signal received by the secondary coil.